Corrosion-resistant coating composition containing hollow microballoons

ABSTRACT

A fluoroelastomer-hollow glass microsphere coating has been found to provide new and unexpected corrosion resistance to metal surfaces for protection against severely corrosive environments. The coating comprises a fluorocarbon elastomer and 3-50% by volume of hollow glass microspheres or microballoons having a size in the range of about 2-300 microns, preferably 20-200 microns. The hollow glass microspheres or microballoons substantially increases the corrosion resistance of a fluoroelastomeric liquid composition, yielding new and unexpected thermal and chemical resistance, particularly when the coating composition is applied in two or more layers.

FIELD OF THE INVENTION

The present invention relates to a coating composition capable ofproviding substrates, particularly metallic substrates, with new andunexpected resistance to corrosion caused by chemicals, particularly athigh temperatures. More specifically, the coating composition of thepresent invention includes a temperature resistant fluoroelastomercontaining 3-50% by volume hollow glass microballoons. The coatingcomposition is particularly desirable for coating the interior surfacesof industrial chimneys, particularly those chimneys where the interiorsurfaces are exposed to hot boiler flue gases containing sulfur, forprotecting metal, interior surfaces against chemically inducedcorrosion. The composition of the present invention is useful forcoating any substrate, particularly a metallic surface, subjected to achemically and/or thermally severe corrosive environment.

BACKGROUND OF THE INVENTION

Flue gases of industrial furnaces commonly contain a severely corrosiveamount of fly ash and sulfur-containing gases and particularly sulfuroxides such as SO₂ and SO₃. When such sulfur containing flue gasescontact an uncoated interior metal surface of a chimney or stack,particularly under high temperatures above about 200° F., the interiorstack surfaces are severely corroded within a short period of time.Accordingly, it is a common practice to wet scrub a flue gas for sulfurremoval not only to meet the governmental pollution standards but alsoto protect the interior stack surfaces. Commonly, in industries such aspower generation stations where it is known that there will be arelatively high percentage of sulfur in the flue gases, and where verytall, i.e. 600 foot stacks are necessary, the interior surfaces of thesestacks may be lined with super stainless steel which will not rust evenunder very extremely corrosive conditions. However, sections of thestainless steel stack linings must be welded, and the weld locationsbecome an extremely desirable attack site for high temperature catalyzedchemical corrosion.

PRIOR ART

A number of patents disclose the use of solid glass fibers or glassspheroids as fillers for polymeric materials: Barnhart et al U.S. Pat.Nos. 2,914,500; Poole 3,036,928; Rising et al 3,056,709; Williams3,249,947; De Fries 3,288,618; McFadden 3,473,952; Iseki et al4,026,863. Further, a number of Veatch et al patents disclose themanufacture of hollow glass spheres as fillers for plastics, concrete,plaster and the like: U.S. Pat. Nos. 2,797,201; 2,978,339; 2,976,840;and 3,030,215. A patent to D'Asto U.S. Pat. No. 3,281,308 discloses thedesirability of coating the interior surface of industrial chimneyswhich are subjected to the combined effects of severely corrosiveenvironments and a relatively high temperature.

SUMMARY OF THE INVENTION

In brief, the present invention relates to a corrosion resistant coatingfor protecting substrate, i.e. metal, surfaces against severelycorrosive environments. The coating comprises a fluorocarbon elastomerand 3-50% by volume of hollow glass microspheres or microballoons havinga size in the range of about 2-300 microns, preferrably 20-200 microns.Fluoroelastomer flexible protective coatings are well known in the artto have outstanding resistance to attack by chemicals, fluids and heat.It has been found that the incorporation of 3-50% by volume of hollowglass microspheres or microballoons into a fluoroelastomeric liquidcomposition, prior to coating, provides a substrate coating with new andunexpected thermal and chemical resistance, particularly when applied intwo or more layers.

Accordingly, an object of the present invention is to provide a new andimproved fluoroelastomeric liquid coating composition containing hollowglass microspheres.

Another object of the present invention is to provide a new and improvedliquid polymeric coating composition, containing 3-50% by volume hollowglass microspheres, having new and unexpected corrosion resistance whencoated over a metal substrate.

Another object of the present invention is to provide a corrosionresistant chimney or flue by coating the interior surface of a chimneyor flue with a composition comprising a fluoroelastomeric, inertheat-resistant polymeric material containing 3-50% by volume of hollowglass microspheres having a diameter in the range of 2-300 microns.

These and other objects and advantages of the present invention will bebetter understood with reference to the following detailed descriptionof a preferred embodiment.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The fluorocarbon elastomers useful in accordance with the presentinvention include any of the fluoroelastomers having sufficienttemperature resistance for the particular environment of use. Generally,the useful fluoroelastomers have molecular weights in the range of10,000-10,000,000 and include any of the polyfluorinated polyolefins andpolyfluorinated polyolefin copolymers, such as trifluorochloroethylene,copolymers of trifluorochloroethylene and vinylidene fluoride;polytetrafluoroethylene; copolymers of tetrafluoro-ethylene andperfluoropropene; copolymers of tetrafluoro-ethylene andchlorotrifluoroethylene; copolymers of tetrafluoro-ethylene with otherfluorinated olefins; and copolymers of hexafluoropropylene andvinylidene fluoride manufactured by E. I. DuPont De Nemours & Companyunder the mark VITON; and mixtures thereof.

The copolymer of hexafluoropropylene and vinylidene fluoride (VITON) iswell known to possess exceptional resistance to oils and chemicals atelevated temperatures. Quite surprisingly, it has been found that theinclusion of hollow glass microspheres in an amount of 3-50% by volumeto a polyolefinic fluoroelastomer, such as the copolymer ofhexafluoropropylene and vinylidene fluoride, substantially increases thechemical resistance of the fluoroelastomer at elevated temperatures.

The hollow glass microspheres useful in accordance with the presentinvention can be any inert, hollow temperature resistant spheres rangingin diameter from about 2 to about 300 microns. It is preferred toincorporate hollow glass (i.e. sodium borosilicate) microspheres havinga diameter in the range of 20-200 microns. Exceptionally good resultshave been obtained when more than 50% by weight of the hollow glassmicrospheres have a diameter in the range of 50-150 microns.

The coating composition of the present invention is prepared bydissolving the fluoroelastomer in a suitable, volatile solvent.Volatile, low molecular weight ketones, such as acetone, areparticularly good solvents for the fluoroelastomers used in the coatingcomposition of the present invention. Generally, the fluoroelastomer isadded to 100-400% by weight of the solvent, based on the weight of thefluoroelastomer, to obtain a suitably fluid, liquid fluoroelastomercomposition for coating. The hollow glass microspheres are added to theliquid solution of fluoroelastomer in an amount of 3-50% by volume offluoroelastomer plus microspheres. The hollow glass microspheres arefragile, thin-walled particles so that mixing of the microspheres intothe liquid fluoroelastomer composition must be done carefully, such asby hand mixing or by closed container rotation, for example on a paintcan roller machine or shaker. During storage, some of the microsphereswill rise to the top of a storage container forming a non-liquid layer.Accordingly, the floating microspheres should be re-suspended in theliquid fluoroelastomer composition immediately prior to use, such as byhand mixing to obtain a generally homogeneous liquid fluoroelastomersolution containing a generally evenly dispersed quantity of hollowmicrospheres.

Prior to coating a metal substrate with the fluoroelastomer-microspherecomposition of the present invention, the metal substrate should bethoroughly cleaned such as by grit blasting and degreasing to ensureadequate coating adhesion. The homogeneous fluoroelastomer compositioncontaining dispersed hollow microspheres is coated over a substrate forexample, by brush, paint roll, swab coater, or other means which willnot fracture the hollow microspheres. It has been found that sprayapplication of the composition of the present invention damages asignificant number of the hollow microspheres so that spray coating isnot recommended. The composition of the present invention should bedried after coating to remove substantially all of the solvent prior toapplication of additional coatings and prior to curing to preventsolvent blistering in the coating layers. The total thickness can beabout 1 to 60 mils, generally 1 to 20 mils in sufficient for corrosionprotection. It has been found that approximately 45 minutes at ambientconditions is sufficient to dry a 2 to 20 mil coating thickness of thecomposition of the present invention when a very volatile, low molecularweight solvent, such as acetone, is used as the fluoroelastomer solvent.

After complete drying of the coatings of the present invention, thefluoroelastomer is cured in any suitable manner, such as by baking thecoating at 400° F. for a period of time of from one to eight hours. Inaccordance with an important feature of the present invention, duringdrying of the composition, the hollow microspheres demonstrate atendency to rise to the exposed surface and the fluoroelastomer shrinksaround the hollow spheroids to provide a thin film of fluoroelastomercompletely encapsulating the outermost exterior of the spheroids. Thisfluoroelastomeric film covering the hollow microspheres remains intactthrough the curing process and during the service life of the coating.The protruding surface layer of encapsulated hollow glass microspheresinsulates and protects the innermost, generally planer layer offluoroelastomer an unexpectedly increases the capability of thefluoroelastomer to protect a metal surface from the combined corrosiveattack of heat and chemicals.

In accordance with an important feature of the present invention, asecond coating layer of the composition of the present inventionunexpectedly improves the performance of the coating composition bynesting the second layer of hollow microspheres, from the secondcoating, within the spaces formed between adjacent, protruding,fluoroelastomer-encapsulated hollow microspheres of the first coatinglayer to provide a metal substrate entirely coated with a heat andchemical resistant, generally planar layer of fluoroelastomer covered bya substantially completely protecting shield formed by an uppermostlayer of fluoroelastomer-encapsulated hollow glass microspheres.

EXAMPLE I

One hundred grams of a copolymer of hexafluoropropylene and vinylidenefluoride is dissolved in 300 grams of acetone to form thefluoroelastomer solution. Then 10 grams of hollow sodium borosilicateglass microspheres having the following particle size distribution anddensity are hand stirred into the fluoroelastomer solution:

    ______________________________________                                        Particle Size Range,                                                          Microns (% by weight)                                                                           > 175  (5)  100-125 (12)                                                      149-175 (10)                                                                               62-100 (44)                                                      125-149 (12)                                                                               44-62  (10)                                                      <44          (7)                                            Average Particle Diameter,        80                                          Microns (weight basis)                                                        Density           .311 grams/cc.                                              ______________________________________                                    

The resulting composition contains about 24.4% by weight fluoroelastomerand about 2.4% by weight hollow microspheres representing approximately30% by volume of the composition after solvent removal.

A steel substrate is first grit blasted and then degreased. A firstcoating of the fluoroelastomer composition of example 1 is brush-appliedand allowed to dry for 45 minutes at ambient temperature. A secondcoating is then brush-applied over the dried first coating and thesecond coating is allowed to dry for 12 hours. The coated substrate isthen baked at 400° F. for 8 hours to affect a complete cure of thefluoroelastomer.

Comparitive testing was performed to determine the effect ofincorporation of the hollow glass microspheres by comparing thecorrosion protecting capabilities of the fluoroelastomer compositionwith hollow glass microspheres to the fluoroelastomer compositionwithout the hollow glass microspheres. Two identical fluoroelastomersolutions were prepared by dissolving equal amounts of a copolymer ofhexafluoropropylene and vinylidene fluoride into acetone. Hollow glassmicrospheres were then hand stirred into one of the fluoroelastomersolutions in an amount of 30% by volume of fluoroelastomer plusmicrospheres. The two fluoroelastomer compositions then were coated on asteel plate using approximately equal amounts of fluoroelastomer in eachcoating, and the coatings were dried and cured. Acid tests of the twocoatings show that the hollow glass microspheres used in accordance withthe present invention approximately double the acid resistance of thesteel substrate coated, as shown in Table I.

                  TABLE I                                                         ______________________________________                                                     Copolymer of                                                                             Copolymer of                                                       Hexafluoro-                                                                              Hexafluoro-                                                        propylene and                                                                            propylene                                                          Vinylidene And Vinylidene                                                     Fluoride   Fluoride And                                                       (Without   30% By Volume                                                      Microspheres)                                                                            Microspheres                                          ______________________________________                                        Coats          2            2                                                 Total Thickness                                                                              2 Mil        4 Mil                                             Bake           1 Hr. at 400° F.                                                                    1 Hr. at 400° F.                           Acid Test Temp.                                                                              500° F.                                                                             500° F.                                    Acid Drops                                                                    (90% H.sub.2 SO.sub.4)                                                                        10           14                                               Time To Failure                                                                              4 Hours      61/2 Hrs.                                         Acid Failure Diameter                                                                        17.5 mm      12.0 mm                                           High Temp.     Over 600° F.                                                                        Over 600° F.                               High Temp. Failure                                                            Area           100.0 mm.sup.2                                                                             53.0 mm.sup.2                                     ______________________________________                                    

The composition of the present invention also was tested in comparisonto a fluoroelastomer composition containing solid glass microspheres.Two identical fluoroelastomer solutions (hexafluoropropylene--vinylidenefluoride copolymer) were prepared in the same manner as the compositionsof Table I. 30% by volume solid glass microspheres were added to onesolution and 30% by volume hollow glass microspheres added to the othersolution. The hollow and solid microspheres had approximately the sameparticle size distribution. In a third, identical fluoroelastomersolution 75% by volume solid glass microspheres were added. Three steelsubstrate surfaces then were grit blasted and degreased and each surfacewas coated with one of the prepared fluoroelastomer compositions, andthe coatings were dried and cured. Acid drops (90% H₂ SO₄) were thenplaced over the coatings and the coatings were observed at ambienttemperatures. After 30 days exposure to 90% H₂ SO₄ it was found that thecomposition containing the hollow glass microspheres is 10 times moreacid resistant then the composition containing an equal volume of solidmicrospheres, as set forth in Table II.

                  TABLE II                                                        ______________________________________                                        BEAD    BEAD VOL.  AVG. DIAMETER AVG. AREA                                    TYPE    RATIO      OF ATTACK     OF ATTACK                                    ______________________________________                                        Hollow  1.0        .281"         .062 sq. in.                                 Solid   1.0        .875"         .601 sq. in.                                 Solid   2.5        1.000"        .786 sq. in.                                 ______________________________________                                    

It was found that the coating having two and one half times the percentby volume of solid glass microspheres was actually more receptive toacid attack.

Steel substrates then were twice coated with the composition of thepresent invention and tested for acid resistance at ambient and elevatedtemperatures. Two coatings approximately two mils each in thickness wereapplied to a steel substrate by brush-applying. The first coating wasallowed to dry at ambient temperature for 45 minutes and the secondcoating was allowed to dry at ambient temperature for 12 hours to assurecomplete drying. After complete drying, the coating was baked at 400° F.for 1 hour to cure the fluoroelastomer. The three compositions set forthin Table II, each comprising a copolymer of hexafluoropropylene andvinylidene fluoride, acetone, and either hollow or solid glassmicrospheres were twice coated on the steel substrate, dryed and cured.The two fluoroelastomer compositions having 30% by volume glassmicrospheres were applied in a thickness of 9 mils whereas thecomposition containing 75% by volume solid glass microspheres wereapplied in a thickness of 14 mils to achieve a coating having about thesame amount of fluoroelastomer. All three coatings were subjected to 90%H₂ SO₄ drops at ambient temperature for 72 hours. The compositioncontaining approximately 30% by volume of solid glass microspheresdeveloped acid attack rings around the acid drops during this 72 hourambient temperature test while the composition containing approximately30% by volume of hollow glass microspheres and the compositioncontaining approximately 75% by volume of solid glass microspheres eachremained inert during the 72 hour ambient test.

The steel substrates twice coated with the composition containingapproximately 30% by volume of hollow glass microspheres (9 mils thick)and the steel substrate twice coated with the composition containingapproximately 75% by volume of solid glass microspheres (14 mils thick)were then subjected to H₂ SO₄ tests at elevated temperatures. The steelsubstrates were subjected to two drops of 90% H₂ SO₄ and the substrateswere heated to 475° F. The substrate coated with the compositioncontaining approximately 75% by volume solid glass microspheres showedthe first sign of failure in the development of a blister in thecoating, while the composition of the present invention containingapproximately 30% by volume hollow glass microspheres only showed aslight darkening during the same time period 2 drops, one hour. The twolayer coating of the composition of the present invention, applied in a9 mil total thickness, did not experience coating failure until 14 dropsof 90% H₂ SO₄ were applied to the coated surface and the surface heatedto 475° F. for a period of 71/2 hours. When the substrate twice coatedwith the composition containing 75% by volume solid glass microspheres,applied in a 14 mil thickness, was subjected to the same conditions 14drops 90% H₂ SO₄ for a 7 and 1/2 hour period at 475° F., vast corrosiondeveloped.

It was also found in acid testing that a steel substrate coated with twocoats of a copolymer of hexafluoropropylene and vinylidene fluoridewithout any glass microspheres develops coating failure in only 1/2 hourwhen the coating is subjected to one drop of 90% H₂ SO₄ at 475° F.

Coating life of the composition of the present invention increases withadded number of coats. It has been found that a second coating of thecomposition of the present invention over a metal substrate increasesthe service life of the coating by about 300% with the third andsubsequent coatings increasing the service life by an averge of 200% peradditional coating, as shown in Table III.

                  TABLE III                                                       ______________________________________                                        Number                            Percent                                     of     Total     Drops Of   Failure                                                                             Increase/                                                                            Test                                 Coats  Thickness 90% H.sub.2 SO.sub.4                                                                     Time  Coat   Temp.                                ______________________________________                                        1      4 Mil     1          1/2 Hr.                                                                             --     500° F.                       2      6 Mil     3          11/2 Hr.                                                                            300%   500° F.                       5      13 Mil     10        6 Hr. 200%   500° F.                       ______________________________________                                    

Surprisingly, it has been found that with two coatings of thecomposition of the present invention over a steel substrate there is nosign of coating failure after three weeks exposure to 60% and 90% H₂ SO₄under ambient temperature, as shown in Table IV.

                  TABLE IV                                                        ______________________________________                                        Temp.    H.sub.2 SO.sub.4 Conc.                                                                   Drops Of Acid                                                                             Time To Failure                               ______________________________________                                        Ambient  60% & 90   One         No Failure After                                                              Three Weeks                                   Ambient  60% & 90%  One         No Failure After                              to 250° F.               Three Weeks                                   450° F.                                                                         60%        24 Drops    23 Hours                                      450° F.                                                                         90%         6 Drops    5.5 Hours                                     500° F.                                                                         60%        37 Drops    19.5 Hours                                    500° F.                                                                         90%         2 Drops    2 Hours                                       550° F.                                                                         60%         4 Drops    2.3 Hours                                     550° F.                                                                         90%         2 Drops    50 Min.                                       600° F.                                                                         60%         2 Drops    1 Hour                                        600° F.                                                                         90%         1 Drop     30 Min.                                       ______________________________________                                    

It should be understood that numerous other modifications andembodiments can be devised by those skilled in the art that will fallwithin the spirit and scope of the principles of this invention.

What is claimed as new and desired to be secured by Letters Patentis:
 1. A substrate coated with a protective coating comprising agenerally planar layer of fluoroelastomer and a plurality of hollowmicrospheres protruding upwardly from said fluoroelastomer layer, eachof said microspheres, being encapsulated in said fluoroelastomer, saidmicrospheres having a particle size in the range of about 2 to about 300microns and included in said fluoroelastomer in an amount of about 3 toabout 50 percent by volume of fluoroelastomer plus microspheres.
 2. Acoated metal substrate as defined in claim 8 wherein saidfluoroelastomer comprises a fluorinated olefinic elastomer.
 3. A coatedsubstrate as defined in claim 1 wherein said fluoroelastomer is acopolymer of two of more fluorinated olefinic polymers.
 4. A coatedsubstrate as defined in claim 1 wherein said fluoroelastomer comprises acopolymer of hexafluoropropylene and vinylidene fluoride.
 5. A coatedsubstrate as defined in claim 1 wherein said microspheres comprisehollow sodium borosilicate glass microballoons in an amount of about 20to about 40 percent by volume of fluoroelastomer plus microballoons. 6.A coated substrate as defined in claim 1 wherein said microspheres havean average diameter of about 50 to about 100 microns.
 7. A coatedsubstrate as defined in claim 1 wherein said substrate comprises metal.8. A coated substrate as defined in claim 1 wherein said substratecomprises a metallic interior portion of a chimney.
 9. A coatedsubstrate as defined in claim 1 wherein said coating is in a thicknessof 1-60 mils.
 10. A coated substrate as defined in claim 1 furtherincluding a second layer of said protective coating, said second layerapplied over a first layer after drying said first layer.
 11. A coatedsubstrate as defined in claim 10 wherein said first and second coatingsare each applied in a dry thickness of 1-20 mils.